Such a process for preparing acids of pyridine bases is known from U.S. Pat. No. 4,482,439. Such acids are valuable as intermediate products for, for instance, corrosion inhibitors, crop protection chemicals and pharmaceutical products. In the specification of that patent it is said, in column 4, lines 51-54, that both cation and anion-exchange membranes can be used, depending on the particular reaction conditions. Reference is made to the examples. In these (20) examples it is always the combination of cation-exchange membrane with acid catholyte or anion-exchange membrane with alkaline catholyte that is used. These combinations have been realized in agreement with the doctrine of Dr. F. Beck as described in his book Elektroorganische Chemie, Grundlagen und Anwendungen, Verlag Chemie (1974). On pages 112-115 of this book it is said that in an electrochemical process carried out in a continuously operating, divided electrolytic cell stationary conditions are to be expected only if in case a cation-exchange membrane is used, the anolyte is acidified, or, in case an anion-exchange membrane is used, the catholyte is alkalized. Only then are the H.sup.+ ions or OH.sup.- ions transferred via the membrane to the other electrode compartment and used up at the counterelectrode to the same degree as they are injected into the first electrode compartment.
In the process as described in the opening lines the chosen anolyte will be acid, because otherwise the alkylpyridines will not be soluble in an aqueous medium. According to the above-mentioned work by Beck, in a divided electrolytic cell the chosen membrane must then be a cation-exchange membrane. A disadvantage of the process described above is that permeation of the protonated alkylpyridine may occur through the membrane from the acid anolyte to the catholyte, upon which this alkylpyridine will subsequently be hydrogenated at the cathode to form the corresponding piperidine derivative. In such a situation there will not only be a loss of raw material, but in many cases also a serious contamination of the cathode will appear.
If the process according to the opening lines is carried out in the presence of an anion-exchange membrane, an alkaline catholyte will be chosen according to Beck. With such a combination of an acid anolyte and an alkaline catholyte it is particularly the situation without any current passing through the cell that may result in an undesired reaction at the membrane (whether anion or cation-exchange) between acid and base. The heat that occurs in such a reaction is apt to result in severe damage of the membrane, particularly if concentrated solutions are used. Moreover, in consequence of diffusion, neutralization occurs also in a current-carrying state, so that acid and base are used up and contamination of the anolyte takes place in consequence of salt formation.